Animation Node Loop with Randomized Color for Each Instance
Your Animation Nodes look fine. I think your issue is with the Cycles NodeTree. There are a couple of issues. First, this connection:



doesn't do what you think it does. Right now, it takes the luminance of the image texture and colors the each pixel of the image based on its luminance. The modification you make to the ColorRamp node with Animation Nodes is overridden. If you want to change the colors, you should use a MixRGB node to mix (or multiply) the ColorRamp output with the image texture:



There is still another problem, though. All of your objects will use the same material, so you can't set the color randomly with Animation Nodes; all of your objects will use the most recent modification of the material since they all point to the same material datablock. 

Instead, you can use the Object Info node in Cycles to give a random color on a per-object basis to your material: